US 2008/0100425 corresponding to JP-A-2008-111809 discloses an occupant detection apparatus having a normal detection mode for determining whether an occupant is seated on a seat by using a capacitive sensor. The occupant detection apparatus further has a fault detection mode for determining whether the capacitive sensor is at fault. In the fault detection mode, a guard electrode of the capacitive sensor is grounded to a vehicle chassis, and a sinusoidal signal is applied by a power source to a main electrode of the capacitive sensor. A calculator calculates an impedance from a detected current or voltage and determines based on the impedance whether the capacitive sensor is at fault.
If the impedance between the main electrode and the guard electrode of the capacitive sensor is low, a load impedance connected to the power source becomes low. In this case, the current flowing in the circuit exceeds a maximum current value to which the power supply can supply the sinusoidal signal without distortion. As a result, distortion occurs in the sinusoidal signal, and a radio noise due to the distortion may be generated in the fault detection mode.
The radio noise may be reduced by connecting a resistor or the like to the circuit to reduce the current flowing in the circuit. In this case, the current varies depending on a resistance of the resistor connected to the circuit. Therefore, there is a concern that the detected current value is so small that it cannot be determined whether the occupant is seated on the seat in the normal detection mode.